pam.conf

, pam.d

- configuration file for pluggable authentication modules

Synopsis

/etc/pam.conf
/etc/pam.d/service

Description

pam.conf is the traditional configuration file for the Pluggable Authentication Module architecture,
or PAM. Per-service policy files in /etc/pam.d/ provide an alternate and preferred
configuration mechanism for PAM.

The PAM library (libpam(3LIB)) looks for the PAM configuration in the following
files in the order listed:

/etc/pam.conf for the current PAM service name

/etc/pam.d/service for the current PAM service name

/etc/pam.conf for the PAM service name of “other”

/etc/pam.d/other

A PAM module provides functionality for one or more of four possible
services: authentication, account management, session management, and password management.

authentication service module

Provides functionality to authenticate a user and set up user credentials.

account management module

Provides functionality to determine if the current user's account is valid. This includes checking for password and account expiration, as well as verifying access hour restrictions.

session management module

Provides functionality to set up and terminate login sessions.

password management module

Provides functionality to change a user's authentication token or password.

Each of the four service modules can be implemented as a shared
library object which can be referenced in the pam.conf configuration file or
a per-service PAM configuration file in /etc/pam.d/.

PAM Configuration File Syntax

The pam.conf file contains a listing of services. Each service is paired
with a corresponding service module. When a service is requested, its associated
module is invoked. Each entry may be a maximum of 256 characters,
including the end of line, and must be one of the following two
formats:

service_namemodule_typecontrol_flagmodule_pathoptionsservice_namemodule_type include path-to-included-PAM-configuration

The per-service policy files in /etc/pam.d/ have almost the same syntax as
pam.conf; however they contain only four fields rather than the five in
pam.conf: the service_name field is not present. The service_name is instead taken
from the name of the policy file in /etc/pam.d/. The two allowed
formats for entries in the per-service policy files are:

module_typecontrol_flagmodule_pathoptionsmodule_type include path-to-included-PAM-configuration

The PAM configuration file included using the include mechanism can be in
any of the four formats listed above.

In both types of PAM policy files blank lines and lines beginning
with a '#' sign are ignored.

The following is an example of a pam.conf configuration file with support
for authentication, account management, session management and password management modules:

service_name denotes the service (for example, login, gdm, or rlogin).

The keyword, “other,” indicates the module that all other applications which have
not been specified should use. The “other” keyword can also be used
if all services of the same module_type have the same requirements.

In the example, since all of the services use the same session
module, they could have been replaced by a single other line in
pam.conf or only the pam_unix_session.so.1 line in /etc/pam.d/other.

The module_path field specifies the relative pathname to a shared library object,
or an included PAM configuration file, which implements the service functionality. If
the pathname is not absolute, shared library objects are assumed to be
relative to /usr/lib/security/$ISA/, and included PAM configuration files are assumed to be relative
to /usr/lib/security/.

The ISA token is replaced by an implementation defined directory name which
defines the path relative to the calling program's instruction set architecture.

The options field is used by the PAM framework layer to pass
module specific options to the modules. It is up to the module
to parse and interpret the options.

This field can be used by the modules to turn on debugging
or to pass any module specific parameters such as a TIMEOUT value.
The options supported by the modules are documented in their respective manual
pages.

Integrating Multiple Authentication Services With Stacking

When a service_name of the same module_type is defined more than once,
the service is said to be stacked. Each module referenced in the
module_path for that service is then processed in the order that it
occurs in the configuration file. The control_flag field specifies the continuation and failure
semantics of the modules, and can contain one of the following values:

binding

If the service module returns success and no preceding required modules returned failures, immediately return success without calling any subsequent modules. If a failure is returned, treat the failure as a required module failure, and continue to process the PAM stack.

definitive

If the service module return success and no preceding required modules return failures, immediately return success without calling any subsequent modules. If a failure is returned, immediately return the first non-optional failure value recorded without calling any subsequent modules. That is, return this failure unless a previous required service module failed. If a previous required service module failed, then return the first of those values.

include

Process the lines from the PAM configuration file that is specified in the module_path at this point in the PAM stack. The ``other'' keyword is used if the specified service_name is not found. 32 levels of included PAM configuration files are supported. Any options are ignored.

optional

If the service module returns success, record the success, and continue to process the PAM stack. If a failure is returned, and it is the first optional module failure, save the failure code as an optional failure. Continue to process the PAM stack.

required

If the service module returns success, record the success, and continue to process the PAM stack. If a failure is returned, and it is the first required failure, save the failure code as a required failure. Continue to process the PAM stack.

requisite

If the service module returns success, record the success, and continue to process the PAM stack. If a failure is returned, immediately return the first non-optional failure value recorded without calling any subsequent modules. That is, return this failure unless a previous required service module failed. If a previous required service module failed, then return the first of those values.

sufficient

If the service module return success and no preceding required modules returned failures, immediately return success without calling any subsequent modules. If a failure is returned, treat the failure as an optional module failure, and continue to process the PAM stack.

If the PAM stack runs to completion, that is, neither a requisite
module failed, nor a binding or sufficient module success stops it, success
is returned if no required modules failed and at least one required,
requisite, optional module succeeded. If no module succeeded and a required or binding
module failed, the first of those errors is returned. If no required
or binding module failed and an optional module failed, the first of
the option module errors is returned. If no module in the stack
succeeded or failed, that is, all modules returned an ignore status, a default
error based on module type, for example, “User account expired,” is returned.

All errors in pam.conf entries or the per-service policy files in /etc/pam.d/
are logged to syslog as LOG_AUTH | LOG_ERR errors. The use of
a service with an error noted in the pam.conf entry for that service
will fail. The system administrator will need to correct any noted errors
before the configured PAM configuration may be used. If no applicable services
are found in the pam.conf file or the per-service /etc/pam.d/ files, the
system administrator may enter system maintenance mode to correct or restore the PAM
configuration.

The following is a sample configuration file that stacks the su, login,
and rlogin services.

In the case of su, the user is authenticated by the inhouse
and authtok_get, dhkeys, and unix_auth authentication modules. Because the inhouse and the
other authentication modules are required and requisite, respectively, an error is returned back
to the application if any module fails. In addition, if the requisite
authentication (pam_authtok_get authentication) fails, the other authentication modules are never invoked, and the
error is returned immediately back to the application.

In the case of login, the required keyword for control_flag requires that
the user be allowed to login only if the user is authenticated
by all the service modules. If pam_unix_auth authentication fails, control continues to proceed
down the stack, and the inhouse authentication module is invoked. inhouse authentication
is optional by virtue of the optional keyword in the control_flag field.
The user can still log in even if inhouse authentication fails, assuming
the modules stacked above succeeded.

In the case of rlogin, the sufficient keyword for control_flag specifies that
if the rhosts authentication check succeeds, then PAM should return success to
rlogin and rlogin should not prompt the user for a password. The
other authentication modules, which are in the stack, will only be invoked
if the rhosts check fails. This gives the system administrator the flexibility
to determine if rhosts alone is sufficient enough to authenticate a remote user.

Some modules return PAM_IGNORE in certain situations. In these cases the PAM
framework ignores the entire entry in pam.conf regardless of whether or not
it is binding, definitive, requisite, required, optional, or sufficient.

Utilities and Files

The specific service names and module types for each service should be
documented in the man page for that service. For instance, the sshd(1M)
man page lists all of the PAM service names and module types for
the sshd command.

The PAM configuration file does not dictate either the name or the
location of the service specific modules. The convention, however, is the following:

pam_module_name.so.x

File that implements various function of specific authentication services. As the relative pathname specified, /usr/lib/security/$ISA is prepended to it.

/etc/pam.conf

Traditional PAM configuration file

/etc/pam.d/service

Alternate PAM configuration files

/usr/lib/$ISA/libpam.so.1

File that implements the PAM framework library

Examples

Example 1 Using the include control flag

The following example collects the common Unix modules into a single file
to be included as needed in the example of a pam.conf file.
The common Unix module file is named unix_common and consists of:

# Authentication management
#
# login service (explicit because of pam_dial_auth)
#
login auth include unix_common
login auth required pam_dial_auth.so.1
#
# rlogin service (explicit because of pam_rhost_auth)
#
rlogin auth sufficient pam_rhosts_auth.so.1
rlogin auth include unix_common
#
# Default definitions for Authentication management
# Used when service name is not explicitly mentioned
#
OTHER auth include unix_common
#
# Default definition for Account management
# Used when service name is not explicitly mentioned
#
OTHER account include unix_common
#
# Default definition for Session management
# Used when service name is not explicitly mentioned
#
OTHER session include unix_common
#
# Default definition for Password management
# Used when service name is not explicitly mentioned
#
OTHER password include unix_common

The equivalent PAM configuration in /etc/pam.d/ would be the following entries in
/etc/pam.d/login:

#
# Default definitions for Authentication management
# Used when service name is not explicitly mentioned
#
auth include unix_common
#
# Default definition for Account management
# Used when service name is not explicitly mentioned
#
account include unix_common
#
# Default definition for Session management
# Used when service name is not explicitly mentioned
#
session include unix_common
#
# Default definition for Password management
# Used when service name is not explicitly mentioned
#
password include unix_common